Maintenance of group call in mobile communication system

ABSTRACT

A method of maintaining a group call in a mobile communication system comprising a network infrastructure and mobile stations, the method comprising the steps of: establishing a group call in which a first mobile station is prioritized and the mobile stations participating in said group call communicate in speech items, the frame structure comprising time slots that a second mobile station sending a speech item employs in its speech item. In the method, the first mobile station is allocated time slots or time periods in which only the first mobile station can transmit signalling messages to the network infrastructure; the first mobile station and the third mobile stations are transmitted data indicating in which time slots of time periods only the first mobile station can send signalling messages to the network infrastructure; and the third mobile stations refrain from sending their signalling messages in the time slots or time periods allocated to the first mobile station.

This application is based on patent application no. 964,817 filed inFinland on Dec. 2, 1996. The content of the Finnish priority applicationis incorporated herein by reference.

FIELD OF INVENTION

The invention relates to a method of maintaining a group call in amobile communication system comprising:

a network infrastructure, and mobile stations that communicate with thenetwork infrastructure over a radio path, whose frame structurecomprises time slots,

the method comprising the steps of:

establishing a group call in which a first mobile station isprioritized, the mobile stations that participate in the group callcommunicating in speech items, and the frame structure comprising timeslots that a second mobile station sending a speech item employs in itsspeech item.

BACKGROUND OF INVENTION

The method of the invention is intended for use particularly in PMR(Private Mobile Radio or Professional Mobile Radio) networks, ortrunking networks, which are typically company networks or privatemobile radio networks used by authorities, all the channels in thesenetworks being allocated to one or more companies or authorityorganizations. In addition to subscriber numbers, the subscribers inthese networks have been assigned group numbers indicating to whichgroup call group or subscriber group the subscriber belongs. It is thuspossible to switch calls directed to the members of a certain group toall the subscribers of this group.

A group call is one of the key functions in a PMR network. A group callis used when several participants are involved, particularly when anentire group must continuously know how things proceed. A group call isa conference call in which all participants can take turns to speak, ortransmit speech items, and to listen to each other. In group calls thewhole group is called by one group identity code. It is known that manymobile telephone systems, particularly systems used by companies andauthorities, apply group calls. As regards the radio path, a group callis typically implemented in simplex form as a point-to-multipoint call,in which speech is transmitted from one talking party to multipletalking parties, and a speech item is allocated to a next talking partyaccording to a predetermined practice. It is naturally also possible toimplement a group call as a duplex call.

A call or a group call is here taken to mean complete exchange ofinformation between two or more parties. A call may consist of one ormore speech items. In a semi-duplex call, these speech items aresequential ones.

The invention is applicable in mobile communication systems with eitherdigital or analogue radio paths. Analogue mobile communication systemsare described, for example, in MPT 1327, A Signalling Standard forTrunked Private Land Mobile Radio Systems, January 1988, revised andreprinted November 1991, and MPT 1343, Performance Specification,January 1988, revised and reprinted September 1991, both issued by theRadio-communications Agency, published by the British Department ofTrade and Industry.

An example of a digital mobile telephone or mobile communication systemin which the invention can be applied is the TETRA system(Trans-European Trunked Radio). Its implementation is described in thestandard ETS 300 392-2, Radio Equipment and Systems (RES);Trans-European Trunked Radio (TETRA); Voice plus Data (V+D) Part 2:Airinterface (AI), Mar. 20, 1996, ETSI, 583 pages. In this system, theallocation of speech items is controlled by a Switching and ManagementInfrastructure (SWMI) through base stations connected to it.

In FDMA systems (Frequency Division Multiple Access), the facility ofsending signalling messages or other data during calls or data calls iscalled in-band signalling. In this system, part of the transmissioncapacity of the channel is typically used during the call for signallingthat supplants actual speech or circuit-switched data.

WO A1 91/09481 describes traffic between a first and a secondcommunication unit in a quasi-duplex situation, for example on a directmode channel. The first communication unit transmits a signal withperiodically-occurring holes therein, the signal also comprising aspeech information portion and information packets that inform of thelocation of the next hole. The receiving, second communication unitsends an interrupt request signal to the first communication unit in thehole indicated by the information packet. The first communication unitreceives the interrupt request signal and processes it, and if the firstcommunication unit determines that the interrupt request signal complieswith (desired) predetermined criteria, it sends an acknowledgement tothe receiving second communication unit. The acknowledgement informs thesecond communication unit that the first communication unit willinterrupt transmission (for a predetermined period of time) and therebyallow the receiving second communication unit to start a transmission.The second communication unit can thus start the transmission.

U.S. Pat. No. 5,025,442 teaches a quasi-duplex mobile telephone systememploying a control time slot during which a receiving communicationunit is allowed to interrupt the transmission of a transmittingcommunication unit by sending the transmitting communication unit aninterrupt signal by which the transmitting communication unit isinstructed not to transmit.

It is also known that part of the traffic capacity allocated to a useris permanently allocated for signalling. This procedure is calledout-band (channel-specific) signalling.

In common mobile telephone networks, such as NMT (Nordic MobileTelephone) or GSM (Global System for Mobiles) systems, typically allcalls have the same priority, i.e. no user is prioritized over any otheruser.

In private radio networks, i.e. trunking networks, the situation isdifferent. The operation of these PMR networks can be arranged in such away that users performing certain type—e.g. emergency—servicing or otherimportant duties have a higher priority, i.e. their high priority callis switched first in the event of congestion.

An example for a high priority call is an emergency call, i.e. a callestablished when at least one subscriber needs immediate help. Thesubscriber in an emergency can activate an emergency call, or theactivation can be performed by some other subscriber station or by adispatcher controlling the operation. The emergency call can beaddressed to a talk group, i.e. a group call, whereby the members of thetalk group need not just listen to the conversation but they can alsoparticipate in it. In other words, the members send speech items. Thisis often necessary, for example, when instructions are given to a groupmember in an emergency. However, the subscriber requiring acommunication connection in an emergency should always be allocated aspeech item as soon as possible in a group call where several users taketurns to speak.

In a mobile telephone system based on a trunking method, the mobilestation, however, has to request a speech item from the system, and thesystem allocates the speech item in order of time or priority. Thesystem is also able to interrupt an ongoing speech item.

A problem in the prior art is that the control channel capacity of aprior art mobile communication system protocol, particularly of theTETRA mobile telephone system, is very limited during speech items inindividual calls of a single base station and in group calls at the basestation that the subscriber station sending a speech item is attachedto. During the speech item, only the 18th frame of the TETRA framestructure has available a Slow Associated Control Channel, i.e. aonce-a-second time slot. Particularly in private mobile radio networkswhich are used by authorities and in which emergency group calls aresent frequently, it is essential that a subscriber or mobile station inan emergency is given a chance to request a speech item sufficientlyoften during a speech item sent by a subscriber participating in a groupcall, so that the speech item request is reasonably likely to besuccessful.

In prior art solutions, particularly in the TETRA system, a SlowAssociated Signalling Channel is used so that all mobile stationsparticipating in a group call can request a speech item using a randomaccess method, such as a first-try procedure, by sending speech itemrequest signals.

A problem arises when a second mobile station is sending a speech itemand several third mobile stations participating in an emergency groupcall, as well as a first mobile station actually in an emergency,simultaneously request a speech item in the time slot of the 18th framereserved for a speech item request, or in a part of the time slot. It isthen likely that it is one of the third mobile stations that succeeds insending a speech item request, rather than the first mobile station,i.e. the one in an emergency. For example, a transmission request from athird mobile station may collide with the speech item request sent bythe first mobile station. The first mobile station, i.e. the one in anemergency, is then not able to deliver its speech item request to thenetwork infrastructure, and so a speech item will not be allocated tothe first mobile station in an emergency.

One way of solving the problem is to allocate a separate uplink channel,time slot or frequency to a prioritized mobile station, so that a speechitem request can be delivered immediately. A problem of this solution isthat it wastes radio resources. It should be noted that when asubscriber is in an emergency or is prioritized, the traffic density isalso otherwise likely to be higher than normal. In addition, allocationof a separate uplink channel to a prioritized user is a complicatednon-standard procedure for a network infrastructure.

Another way of solving the above problem is to allow a prioritizedmobile station to send a speech item request on a Common ControlChannel. A problem is that a prioritized mobile station has to wait forits transmission turn on the control channel, and that in so doing theprioritized mobile station is not on a traffic channel, whereby it maylose a call or at least not receive part of the transmission of atransmitting mobile station.

Another problem in the prior art solutions is that when a speech item isterminated in a prioritized group call, there begins a random accesstime, during which the mobile stations participating in the group callcan request a speech item. It may then happen that the speech itemrequests of the third mobile stations collides with the speech itemrequest made by the first, prioritized mobile station, and the firstmobile station cannot therefore be quickly allocated the speech item itneeds. This has particular significance in private mobile radio networksused by authorities, where it must be ensured that a speech item requestfrom a prioritized mobile station, potentially in an emergency, isreasonably likely to succeed.

BRIEF DESCRIPTION OF INVENTION

The object of the present invention is to solve the above problem of theprior art.

The object of the present invention is to provide a method by which itcan be ensured that speech item requests sent by a prioritized mobilestation, e.g. a mobile station in an emergency, participating in aprioritized call actually reach the network infrastructure, which canallocate a speech item precisely to the prioritized mobile station afterreceiving the speech item request.

This new method of maintaining a group call is achieved with a method ofthe invention, the method being characterized by further comprising thesteps of:

allocating only to the first mobile station time slots or time periodsin which only the first mobile station may send signalling messages tosaid network infrastructure,

sending the first mobile station and the third mobile stations dataindicating in which time slots or time periods only the first mobilestation may send signalling messages to said network infrastructure,

the third mobile stations refraining from sending their signallingmessages in said time slots or time periods allocated to the firstmobile station.

The invention also relates to a mobile communication system comprising:

a first, a second and third mobile stations, a mobile exchange forrelaying group calls and transmissions and for maintaining a group call,said mobile stations that participate in said group call communicatingin speech items,

a base station for maintaining a frame structure comprising time slotsthat the second mobile station sending a speech item employs in itsspeech item.

The mobile communication system of the invention is characterized byfurther comprising:

allocation means for allocating time slots or a time period only to thefirst mobile station so that only the first mobile station can sendsignalling messages in certain time slots or time periods to said mobileexchange,

transmission means for sending the first mobile station and the thirdmobile stations data in a control message indicating in which time slotsor time periods only the first mobile station may transmit,

control means in said third mobile stations for preventing transmissionof signalling messages in said time slots or time periods allocated tosaid first mobile station.

The invention also relates to a mobile station of a mobile communicationsystem, communicating in a group call, the mobile station comprising: atransceiver for sending transmissions in a frame structure comprising:time slots that a second mobile station sending a speech item in thegroup call employs in its speech item, and time periods that are commonto the mobile stations that are not sending a speech item, fortransmitting signalling messages to the network infrastructure; acontrol unit; a user interface; and a memory unit.

A mobile station of the invention is characterized in that said mobilestation comprises a control unit, responsive to a control message sentby said mobile communication system, for preventing transmission ofsignalling messages in the time slots or time periods allocated to afirst mobile station and indicated in said control message.

A mobile station of the invention can also be characterized in that, inresponse to a control message sent by said mobile communication system,the transceiver of the mobile station is arranged to send a signallingmessage during the time period indicated in the control message.

The invention is based on the idea of ensuring that a participant of aprioritized group call, for example a mobile station that has activatedan emergency call, is assigned signalling capacity at certain intervals,so that said mobile station has a chance of sending a speech itemrequest, and on the other hand so that the mobile station sends thesystem an indication of its presence on a continuous basis. This is doneby allocating a random access time slot of the frame structure to thefirst, prioritized mobile station, and by preventing the other, thirdmobile stations that participate in the same call from transmittingduring a time period of the frame structure that is allocated to thefirst mobile station. In the invention, the first, prioritized mobilestation, which is the one in an emergency, and the third mobilestations, participating in the prioritized group call, are sent dataindicating the time slots or time periods in which only the first mobilestation is allowed to send signalling messages to said networkinfrastructure.

The advantage of the method, mobile communication system and mobilestation of the invention is that the invention ensures that, forexample, a mobile station in an emergency, particularly a prioritizedmobile station, can send a speech item request, and that when the mobilestation sends the speech item request, the request also reaches thenetwork infrastructure, which is then able to allocate the next speechitem to the prioritized mobile station, for example to one in anemergency.

BRIEF DESCRIPTION OF FIGURES

In the following the invention will be described in greater detail withreference to the attached drawings, in which

FIG. 1 shows a frame structure of the TETRA mobile telephone system,

FIG. 2 shows a timing diagram of a group call prioritized according to afirst embodiment of the invention,

FIG. 3 shows a timing diagram of a group call prioritized according to asecond embodiment of the invention,

FIG. 4 shows a block diagram of a mobile station of the invention,

FIG. 5 shows a schematic view of a situation in which a prioritizedmobile station is given a chance of transmitting position data atfrequent intervals in accordance with the invention,

FIG. 6 shows a block diagram of a mobile communication system of theinvention.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows a frame structure of the TETRA mobile telephone system. Ina TETRA protocol, the uplink frequency (from mobile station to basestation) on system channels is delayed by two time slots from thedownlink (from base station to mobile station). The system comprisesmain radio frequencies with a main signalling channel and possible othersignalling channels and traffic channels.

In the TETRA frame structure, one hyper frame comprises 60 multiframes,each one of which comprises 18 TDMA frames. Each TDMA frame is dividedinto four time slots, 101 to 104 in the figure. According to the TETRAstandard, a time slot or part of a time slot, a time period, for examplehalf a time slot, in the 18th frame of a multiframe is allocated as anuplink signalling channel, and the other time slots of the frameconcerned, for example 101, 103 and 104, can be allocated as trafficchannels. In the figure, multiframes are indicated by 1 to 60, frames by1 to 18, and time slots by 1 to 4.

FIG. 2 shows a timing diagram of a group call prioritized according to afirst embodiment of the invention. At the beginning of the operation auser indicates an emergency to the system. The user giving theindication can himself/herself be in an emergency, or he/she can be, forexample, a dispatcher detecting that a mobile station requested toparticipate in a group call is in an emergency. The system operates inaccordance with an emergency call procedure: for example, it allocates aspeech item to the mobile station that activated the emergency call, androutes the call to an emergency exchange. The method of the inventionrelates to a situation where an emergency call turns into a group callin which other members of the group also have a chance to speak. Whensome other mobile station of the talk group than the one that is in anemergency and is to be prioritized, i.e. for example a second mobilestation, sends a speech item 201, 202, 203, then the one in anemergency, i.e. the first, prioritized mobile station, must be able topre-empt a group member's speech item, if necessary.

FIG. 2 shows, hatched, the successive time slots of a certain uplinkconnection or channel in which the second mobile station sends itsspeech item 201, 202, 203. In FIG. 1, for example, the time slots in thefirst seventeen frames of a multiframe are the same time slot, forexample time slot 102. The pre-emption of a prioritized mobile stationis implemented as follows: the system notifies the prioritized mobilestation and the third mobile stations, i.e. the other mobile stationslistening to the speech item of the second mobile station, that thefirst mobile station has been allocated a time slot or time period 204,205 in which the mobile station can transmit a speech item request. Thenotification is performed by transmitting a signalling message at leastto the first mobile station and the third mobile stations. In responseto the notification, the third mobile stations refrain from transmittingtheir signalling messages in the time slots or time periods allocated tothe first mobile station.

In accordance with the present invention, the first mobile station canbe required to send an empty transmission, for example a Null PDU(Protocol Data Unit), if it has nothing else to send in the time slot ortime period 204, 205 concerned, i.e. no speech item request or any othersignalling message. This makes it possible to achieve the third objectof the invention: a prioritized mobile station, for example one in anemergency, is forced to announce its existence on a continuous basis.

If necessary, the first mobile station can send positioning data in thetime slot or time period 204, 205 allocated to it. The geographicallocation of the mobile station concerned can then be determined on thebasis of the data. The positioning data can be, for example, GPS (GlobalPositioning System) position coordinate data. On the basis of theposition data, the positioning system connected with, for example, amobile communication system or its dispatcher can announce the locationof the first, prioritized mobile station every time the mobile stationsends location data. A change in the location of the first mobilestation, i.e. its movements, can thus be monitored.

The part of a frame structure allocated to the first mobile station fortransmission of speech item requests can be a whole time slot, a timeperiod consisting of several time slots or one part of a time slot.

The method according to the first embodiment of the invention comprisesat least the following steps:

At first a group call is established so that the frame structure of themobile communication system comprises, between normal communication timeslots, time periods that are common to the first mobile station and thethird mobile stations for transmission of signalling messages to thenetwork infrastructure. This concerns a situation in which the secondmobile station is sending a speech item, and the first, prioritizedmobile station wants to be allocated the next speech item.

In the allocation step that follows, one or more of the common timeperiods are allocated only to the first mobile station for transmissionof signalling messages. It is thus decided that a time slot of certainframes, for example of the 18th frames in the frame structure, isallocated to be used only by the first mobile station for transmissionof signalling messages.

The network infrastructure sends the first mobile station and the thirdmobile stations a notification of the allocation, and in response to thenotification the third mobile stations refrain from sending theirtransmissions during the time period concerned.

FIG. 3 is a timing diagram of a group call prioritized in accordancewith a second embodiment of the invention. The embodiment is amodification of the above-described first embodiment, and all that isstated above in connection with the first embodiment also applies tothis embodiment, unless specifically stated otherwise. The secondembodiment concerns a situation where the second mobile station has usedup its speech item 301 and terminated it 302. In the prior art, when aspeech item is terminated, the mobile stations listening to the channelare usually allowed random access. The mobile stations listening to thechannel can then send speech item request sianals or other signallingmessages. The aim is allocation of a new speech item. A prioritizedmobile station is in a special situation in the respect that if it needsto initiate a speech item, it must be allowed to initiate it as soon aspossible. The reason for this is that the prioritized mobile station canbe in an emergency situation and need help, and is therefore requestinghelp. The prioritized mobile station may also have requested that anemergency call should be set up.

FIG. 3 shows a situation where pre-emption becomes possible for allmobile stations of a group call immediately as the preceding, second,mobile station has terminated its speech item. The pre-emption situationis brought about as follows: the network infrastructure of the mobilecommunication system allocates a certain number of first time slots ortime periods 304 to the first mobile station and at least the thirdmobile stations participating in the group call for transmission ofrandom access and signalling messages. In addition, the system allocatessecond time slots 305 of the frame structure only to the first, orprioritized, mobile station, for transmission of signalling messages,typically of a speech item request. The first, or prioritized, mobilestation and the third mobile stations participating in the group callare notified of the allocation. The second mobile station, which hasterminated a speech item, can also be notified. The notification of theallocation contains at least data indicating in which time slots or timeperiods only the first mobile station can send signalling messages. To aprioritized mobile station optionally in an emergency is allocated atime slot or time period, e.g. half a time slot, in which only thisprioritized mobile station can send a speech item request. This isimplemented in such a way that it is possible for a prioritized user,for example a user in an emergency, not to want to pre-empt a precedingcall and yet want to be allocated the next speech item.

If the first mobile station were allocated the first free time slot ortime period after the second mobile station has terminated its speechitem 302, then the pre-emption situation 303 would occur too early,since the user's reaction to the termination of a speech item isslightly delayed (e.g. half a second) from the moment when the userdetects the allocation of a time slot. It is naturally possible toallocate a time slot to the first mobile station immediately as thespeech item of the second mobile station has been terminated, or in anyother free time slot or time period.

A next speech item 306 follows the time slot or time period 305allocated to the first mobile station for transmission of signallingmessages. If the first, prioritized mobile station has requested aspeech item, it may be allocated the next speech item 306. Otherwisesome other mobile station that has requested a speech item is allocatedthe next speech item 306. After the next speech item, there follows anew time slot or time period 307 of the frame structure, in which anymobile stations except the one sending a speech item can transmit theirsignalling messages to the network infrastructure. If so defined in thedata sent by the network infrastructure, even this time slot or timeperiod can still remain allocated to the first mobile station fortransmission of signalling messages.

FIGS. 2 and 3 show the use of several random access frames forallocation of speech items. After the last speech item, a time slot isreserved for the prioritized mobile station for transmission of a speechitem request. The random access frame can be a TDMA time slot or a partof such a time slot.

In the second embodiment of the invention, at least the following stepsare taken. When the second mobile station, which has been sending aspeech item, terminates its speech item, the third mobile stationsparticipating in the group call are allocated, for transmission ofsignalling messages, a desired number or time slots or time periods froma time period of the frame structure that is of a desired length.

In addition to this, from said time period of a desired length isallocated a time period only to the first mobile station fortransmission of signalling messages. These signalling messages can be,for example, speech item request signals, status messages, messagesinforming of the location of the mobile station, or empty messages.

After this, the mobile stations participating in the group call are senta first notification to the effect that certain time slots or timeperiods of the frame structure are allocated to the mobile stationsparticipating in the group call. The notification informs the mobilestations participating in the group call that certain time slots areavailable to all the mobile stations of the group for transmission ofsignalling messages.

The first mobile station is then sent a second notification to theeffect that a certain second time period of the frame structure isallocated only to the first mobile station for transmission ofsignalling messages. Naturally, there can be more than one first mobilestation.

In response to the first notification, the mobile stations participatingin the group call can then send a signalling message during theabove-mentioned time slots or time periods.

Further, in response to the second notification, the first mobilestation can send a transmission during the time period allocated to it.

In the first and second embodiments of the invention, in response to theallocation data sent by the network infrastructure, the third mobilestations store in their memory data indicating which time period of theframe structure is allocated to the first mobile station.

FIG. 4 is a block diagram of a mobile station of the invention. FIG. 4shows a typical communicating radio unit 400, i.e. a mobile phone,mobile station or e.g. a subscriber station used by the subscriber. Thefunction of a transceiver (TX/RX) 401 is to tune to the channel used.The transceiver 401 communicates with base stations or repeater stationsat the radio frequency, i.e. a radio frequency divided into TDMA framesand further into time slots, at least one of which can be allocated toone or more mobile stations as a traffic channel. To the transceiver 401is connected an antenna 402, which is connected with the radio path RP.Usually, radio frequencies in the range of 60 to 1000 MHz (VHF and UHFranges) are used, but other frequencies are also possible. On the radiopath RP, analogue modulation can be used, the modulation then beingusually phase modulation. Other kinds of modulation can also be used.Signalling can be transmitted e.g. by a voice-frequency subcarrier wave(FFSK). Transmission over a radio path can also be digital. The radiounit can tune to and communicate at uplink and downlink frequencies.

A user interface 405 comprises electroacoustic transducers, typically aheadphone 406 and a microphone 407, and optionally buttons for startingand ending a call, and for dialling. Since transmission over the radiopath RP is advantageously unidirectional in a trunked system, thesubscriber station usually also has a push-to-talk button that must bedepressed for the duration of the speech item. The push-to-talk buttonis not shown in FIG. 4.

The function of a controller 403 is to control the operation of theradio unit. The controller 403 is connected to the user interface 405,from which it receives impulses e.g. for starting and ending a call. Thecontroller 403 can also give the user, via the user interface 405,acoustic or visual signals that relate to the operation of the mobilephone and/or the mobile telephone system.

The controller 403 is connected to the transceiver TX/RX 401. Thechannel used by the transceiver is allocated by the controller 403, i.e.the transceiver 401 tunes to the channel, or radio frequency, and asuitable time slot, allocated by the controller 403. The transceiver 401is also activated by the controller 403. The controller 403 receives andtransmits signalling messages through the transceiver 401. Acommunicating radio unit or mobile station 400 of the invention can beused e.g. in a radio system that comprises a radio network with at leastone base station, mobile stations, and optionally one or more repeaterstations that forward traffic between at least one base station and thesubscriber stations. The communicating mobile station here comprises amemory means 409, a transceiver 401, and a controller 403 forcontrolling the operation of the mobile station.

The invention relates to a mobile station of a mobile communicationsystem communicating in a group call, the mobile station comprising atransceiver for sending transmissions in a frame structure thatcomprises: time slots that are employed by the second mobile stationsending a group call speech item in its speech item, and time periodsthat are common to mobile stations that are not sending a speech item,for transmission of signalling messages to the network infrastructure.

In the following we shall primarily describe a mobile station thatoperates in the same way as the third mobile stations described above inconnection with the method steps operate. The mobile station of theinvention comprises a control unit C1, responsive to a control messagesent by the mobile communication system, for preventing transmission ofsignalling messages in the time slots or time periods allocated to thefirst mobile station and indicated by said control message.

In response to a control message sent by the network infrastructure, thememory unit 409 of the mobile station is arranged to store in its memorymeans 409 data indicating that a certain time period of the framestructure of the system is allocated to some other mobile station, i.e.to the first mobile station in the method steps.

In the following we shall primarily describe a mobile station thatoperates in the same way as the first mobile station described above inconnection with the method steps operates.

In response to a control message sent by the mobile communicationsystem, the transceiver 401 of this prioritized mobile station isarranged to send its signalling message during a time period indicatedin the control message.

In response to a transmission prompt sent by the mobile communicationsystem, the transceiver 401 of this prioritized mobile station isarranged to start sending its transmissions in the time slots indicatedby the transmission prompt contained in the control message.

FIG. 5 is a diagram illustrating a situation in which, in accordancewith the invention, a priorized mobile station 501 is given a chance totransmit position data at frequent intervals. In a mobile communicationsystem it may happen that a mobile station 501 loses a call. Inparticular, this may happen if transmission trunking orquasi-transmission trunking is used for channel allocation in the mobilecommunication system. Because of this, if one wants to ensure that amobile station in an emergency really participates in a call all thetime, then the mobile station must be required to send a presenceindication at regular intervals. If it is found out that the mobilestation is not participating in the call, then the mobile communicationsystem, typically the exchange of the system, can start informing of thecall on a control channel, since a mobile station that has withdrawnfrom a call is likely to return to the control channel after losing thecall. In addition, the periodic indication of the presence isoperatively useful, for example, since it makes it possible to inform adispatcher listening to an emergency call that the radio connection withthe mobile station of a subscriber in an emergency has not been lost.

If a positioning means (AVL-GPS), GPS (Global Positioning System) isavailable, its transmission frequency can be increased in an emergencyby using this invention, so that any changes in the location of themobile station are recorded accurately. A signalling instant allocatedto a mobile station can be used for transmitting this kind ofinformation when it is not necessary to use the signalling instant forpre-emption. The difference between the transmission of location data ina standard situation and in an emergency is shown in FIG. 5. It appearsfrom the figure that, if desired, it is possible to give the mobilestation a chance to send its position data to the system more often whenit is participating in a group call. This is particularly important whenthe mobile station is, for example, in an emergency.

FIG. 6 is a block diagram of a mobile communication system of theinvention. The invention relates to a mobile communication systemcomprising a first M1 and a second M2 mobile station and third M3 mobilestations, and network infrastructure INFRA comprising an exchange MX forforwarding group calls and transmissions and for maintaining a groupcall GC, the mobile stations M1, M2 and M3 that participate in the groupcall communicating in speech items, and a base station BS formaintaining the frame structure, which comprises time slots employed bythe second mobile station M2, which is sending a speech item, in itsspeech item. The inventive mobile communication system further comprisesallocation means A for allocating time slots or a time period only tothe first mobile station M1, so that only the first mobile station M1can send signalling messages in certain time slots or time periods tosaid network infrastructure.

The mobile communication system of the invention further comprisestransmission means T for sending the first mobile station M1 and thethird mobile stations M3 data in a control message indicating in whichtime slots or time periods only the first mobile station M1 cantransmit. The inventive mobile communication system further comprisescontrol means R in the third mobile stations for preventing transmissionof signalling messages in the time slots or time periods allocated tothe first mobile station M1.

The frame structure of the mobile communication system comprises timeperiods that are common to the first mobile station M1 and the thirdmobile stations M3 for transmission of signalling messages to the basestation BS.

In the inventive mobile communication system, allocation means A arearranged to allocate a certain time period of the common time periodsonly to the first mobile station M1 for transmission of signallingmessages, and transmission means T are arranged to send the third mobilestations M3 a control message to the effect that a time period of theframe structure is allocated to the first mobile station M1.

The inventive mobile communication system further comprises controlmeans C, responsive to a signalling message transmitted by the firstmobile station M1, so as to instruct the second mobile station M2 toterminate its speech item, and transmission means T2, responsive to asignalling message transmitted by the first mobile station, so as tosend a transmission prompt to the first mobile station.

The drawings and the above description are intended only to illustratethe idea of the invention. The method, mobile communication system andmobile station of the invention can vary in their details within thescope of the claims. Although the invention is described above mainly inconnection with the TETRA mobile communication system, the invention canbe used in other kinds of mobile communication systems as well.

We claim:
 1. A method of maintaining a group call in a mobilecommunication system comprising a network infrastructure, and mobilestations that communicate with the network infrastructure over a radiopath, whose frame structure comprises time slots, the method comprising:establishing a group call in which a first mobile station isprioritized, the mobile stations that participate in the group callcommunicating in speech items, and the frame structure comprising timeslots that a second mobile station sending a speech item employs in itsspeech item; allocating only to the first mobile station time slots ortime periods from the frame structure of said group call in which timeslots or time periods only the first mobile station may send signallingmessages to said network infrastructure; and sending the first mobilestation and a third mobile station data indicating in which time slotsor time periods only the first mobile station may send signallingmessages to said network infrastructure, the third mobile stationrefraining from sending signalling messages in said time slots or timeperiods allocated to the first mobile station.
 2. The method accordingto claim 1, wherein said establishing of said group call is performed sothat said frame structure further comprises time periods that are commonto the first mobile station and the third mobile station and are usedfor transmission of signalling messages to the network infrastructure;allocating, in said allocation step, one or more of said common timeperiods only to the first mobile station for the transmission ofsignalling messages; transmitting, in said transmission step, allocationdata from said network infrastructure to the first mobile station andthe third mobile station to the effect that said time periods of saidframe structure are allocated to the first mobile station; and inresponse to said allocation data, the third mobile station refrainingfrom sending transmissions during said time period.
 3. The methodaccording to claim 2, in response to the second mobile stationterminating the speech item, the method further comprises: allocating,in said allocation step, after the termination of said speech item ofthe second mobile station, a desired number of time slots or timeperiods from a time period of the frame structure that is of a desiredlength to said third mobile station participating in the group call fortransmission of signalling messages; allocating, in said allocationstep, a certain time period from said time period of a desired lengthonly to the first mobile station for transmission of signallingmessages; transmitting, in said transmission step, to the mobilestations participating in said group call a first notification to theeffect that said time slots or time periods of said frame structure areallocated only to the mobile stations participating in said group call;transmitting, in said transmission step, to the first mobile station asecond notification to the effect that said certain time period of saidframe structure is allocated to the first mobile station, wherein inresponse to the first notification, the mobile stations that participatein said group call being able to send a signalling message during saidtime slots or time periods, and in response to the second notification,the first mobile station being able to send its transmission during saidtime period.
 4. The method according to claim 3, wherein in response tothe first notification, the mobile stations that participate in saidgroup call store data indicating which time slots or time periods ofsaid frame structure are allocated to the mobile station participatingin said group call.
 5. The method according to claim 3, wherein inresponse to the second notification, the first mobile station stores inmemory data indicating which time period of said frame structure isallocated to the first mobile station.
 6. The method according to claim2, wherein the third mobile station, in response to said allocationdata, stores information in memory to the effect that said time periodof said frame structure is allocated to the first mobile station.
 7. Themethod according to claim 2, further comprising: instructing the secondmobile station to terminate the speech item in response to saidsignalling message sent by the first mobile station; sending atransmission prompt to the first mobile station; and sending firstmobile station transmissions in time slots indicated by the transmissionprompt.
 8. The method according to claim 1, wherein said time periodsare time slots of said frame structure, or parts of such time slots. 9.The method according to claim 1, further comprising: transmitting saidallocation data or second notification to the first mobile station,wherein in response to said allocation data or second notification, thefirst mobile station being able to transmit a signalling message to saidnetwork infrastructure during said time period.
 10. The method accordingto claim 1, wherein said signalling messages are speech item requestsignals.
 11. The method according to claim 1, wherein said signallingmessages are packet form data.
 12. The method according to claim 11,further comprising: transmitting, by the first mobile station, part ofthe data to be transmitted in said signalling message, whereby part ofthe data to be transmitted does not fit into the first time period used;requesting the network infrastructure to allocate more time to the firstmobile station in said frame structure for transmission of the remainingdata from the first mobile station to the network infrastructure;allocating additional time from said frame structure to the first mobilestation in the network infrastructure for transmission of the remainingdata; and transmitting a notification of the allocated additional timeto the first mobile station and the third mobile station.
 13. The methodaccording to claim 12, wherein a desired time period is allocated fromsaid frame structure to the first mobile station for transmission of theremaining data.
 14. The method according to claim 12, wherein one ormore time slots are allocated from said frame structure to the firstmobile station for transmission of the remaining data.
 15. The methodaccording to claim 1, wherein said signalling messages are statusinformation messages.
 16. The method according to claim 1, wherein saidsignalling message comprises position data indicating the location ofthe first mobile station.
 17. A mobile communication system comprisingfirst, second and third mobile stations, a mobile exchange for relayinggroup calls and transmissions and for maintaining a group call, saidmobile stations that participate in said group call communicating inspeech items, a base station for maintaining a frame structurecomprising time slots that the second mobile station sending a speechitem employs in a speech item, the mobile communication systemcomprising: allocation means for allocating time slots or a time periodfrom the frame structure of said group call, so that only the firstmobile station can send signalling messages in certain time slots ortime periods to said mobile exchange; a first transmission means forsending the first mobile station and the third mobile station data in acontrol message indicating in which time slots or time periods only thefirst mobile station may transmit; and a first control means in saidthird mobile station for preventing transmission of signalling messagesin said time slots or time periods allocated to said first mobilestation.
 18. The mobile communication system according to claim 17,wherein said frame structure comprises time periods that are common tothe first mobile station and the third mobile station for transmissionof signalling messages to said base station, wherein said allocationmeans are arranged to allocate a certain time period of said common timeperiods only to the first mobile station for transmission of signallingmessages, and said transmission means are arranged to send the thirdmobile station a control message to the effect that said time period ofsaid frame structure is allocated to the first mobile station.
 19. Themobile communication system according to claim 17, further comprising: asecond control means, responsive to said signalling message sent by thefirst mobile station, for instructing the second mobile station toterminate the speech item; and a second transmission means, responsiveto said signalling message sent by the first mobile station, for sendinga transmission prompt to the first mobile station.
 20. A mobile stationof a mobile communication system communicating in a group call, themobile station comprising: a transceiver for sending transmissions in aframe structure comprising time slots that a second mobile stationsending a speech item in a group call employs in a speech item, and timeperiods that are common to the mobile stations that are not sending aspeech item, for transmitting signalling messages to a networkinfrastructure; a first control unit; a user interface; a memory unit;and a second control unit, responsive to a control message sent by saidmobile communication system, for preventing transmission of signallingmessages in the time slots or time periods allocated to a first mobilestation as indicated in said control message.
 21. The mobile stationaccording to claim 20, wherein in response to said control message, saidmemory unit is arranged to store information to the effect that saidtime period of said frame structure is allocated to some other mobilestation.
 22. A mobile station of a mobile communication system,communicating in a group call, the mobile station comprising: atransceiver for sending transmissions in a frame structure comprisingtime slots that a second mobile station sending a speech item in a groupcall employs in a speech item, and time periods that are common to themobile stations that are not sending a speech item, for transmittingsignalling messages to the network infrastructure; a control unit; auser interface; a memory unit, wherein in response to a control messagesent by said mobile communication system, the transceiver of the mobilestation is arranged to send a signalling message during the time periodof the frame structure of said group call indicated in the controlmessage.
 23. The mobile station according to claim 22, wherein inresponse to a transmission prompt sent by said mobile communicationsystem, said transceiver is arranged to start sending transmissions intime slots indicated in said transmission prompt.